In leukemias, non-random chromosomal translocations have been determined to predispose or cause changes in cell growth and/or survival that promote malignancy. The detailed study of the mechanisms of human cancer causing genes and their protein structure requires the identification or development of animal models to allow the study of protein function. Patients with the disease ataxia telangiectasia (AT) frequently suffer from T cell chromic lymphocytic leukemia (T-CLL) carrying a 14q32.1-q11 inversion/translocation. The chromosomal breakpoint gene Tcll and the related MTCP1 genes have been recently cloned and characterized and the gene responsible for their underlying disease (ATM) has been cloned and a null mutation produced in mice (Atm-/-). We hypothesize that Tcll plays a critical role in the development of cancer. Further the use of the newly isolated murine Tcll gene for the development of a mouse model of lymphoproliferation will provide an ideal opportunity to study the Tcll protein and its role in inducing mature T cell lymphoproliferations that lead to CLL. Consequently, we propose three experimental aims: 1 Evaluate the expression of Tcll during murine embyrogenesis. The spatial expression of Tcll will be critical for null mutant analysis and functional studies. 2 Develop and analyze Tcll transgenic and Tcll -/- null mutant mice. These strains will be required for the full development of an animal model of human disease. 3 Establish a mammalian model of T cell lymphoproliferation and CLL. Tcll -/- mutant mice will be bred with other mouse strains, such as the Atm +/- mice to recapitulate human CLL. We expect that through the development of these molecular tools, including a murine model of CLL, therapies for the human disease can be developed and tested.